1. Field of the Invention
This invention provides a method and related apparatus for checking the reliability of gas detection instruments through the generation of a test gas and its application to the gas detection instrument.
2. Description of the Prior Art
The reliability of toxic gas detectors is of great importance in many applications, especially when these instruments are used for ensuring the safety of personnel. Reliability is typically obtained by periodic checking of the instrument response to a test gas, however calibration test gases are typically supplied in large, bulky and expensive gas cylinders.
Potentially hazardous atmospheres are found in many locations, due to the presence of toxic gases, combustible gas mixtures or the excess or deficiency of oxygen concentration. Many types of gas detection instruments have been developed to provide a warning that the atmosphere contains potentially hazardous components, or to initiate remedial action. Examples of these gas detection instruments include the detection of combustible gases (primarily methane) in coal mines, hydrogen sulfide in oil fields and water treatment plants, carbon monoxide in places ranging from steel mills to bedrooms, and oxygen in confined spaces, such as sewers. Within each gas detection instrument there are one or more gas sensors, whose function is to provide an electrical signal, which varies in response to the gas concentration.
Many types of sensor technology are used for gas detection, including electrochemical, infrared, catalytic bead (heat of combustion), and tin oxide sensors. Details of these various sensor types are discussed in standard texts such as C. F. Cullis, J. G. Firth, xe2x80x9cDetection and Measurement of Hazardous Gasesxe2x80x9d, Heinemann, London, 1981; P. T. Mosely, J. O. W. Norris, D. E. Williams (Eds.), xe2x80x9cTechniques and Mechanisms in Gas Sensingxe2x80x9d, Adam Hilger, Bristol, 1991. Each of these various technologies have different advantages and weaknesses, such that the method used will depend on the gas to be detected and the application requirements.
In general, most gas sensors provide a relative output signal, such that the output signal is not an absolute measure of the gas concentration. Instead the response is typically proportional to the gas concentration, with an empirically determined proportionality constant. Before the instrument can be used to measure the concentration of a gas, the instrument is first exposed to a known test gas concentration and the output signal correlated with the known gas concentration. This process is known as calibration.
Another role of calibration is to provide a function check to confirm that the gas detection instrument is operating correctly. Unfortunately, the output from many types of sensors can vary over time and in some cases sensors can fail to operate correctly without warning. It is therefore desirable to re-calibrate the sensor periodically. The interval between calibrations will depend on the sensor technology and on the accuracy requirements of the application. For example, electrochemical gas sensors, which are widely used for toxic and oxygen gas detection in industrial work place safety applications, are typically re-calibrated monthly; while many infrared combustible gas sensors may only require calibration every six months or every year.
Calibration is often a time consuming process, but for critical applications such as safety monitoring, a more frequent calibration interval may be used than is employed for a less critical application. One method commonly employed to reduce the burden of calibration is to perform a so-called xe2x80x9cbump testxe2x80x9d, in which the instrument is exposed to a test gas of sufficiently high concentration to activate the warning alarms for a short period of time. If the instrument alarms are actuated, then the instrument is deemed to be working correctly. However, if the instrument alarms do not actuate, then the instrument requires servicing. While calibration of the instrument is usually performed with test gases with concentrations known to a high degree of accuracy, bump tests are often performed with a more economical test gas mixture whose concentrations are known to a lower degree of accuracy.
Test gases are commonly available in compressed gas cylinders. For everyday use, small hand-held, disposable gas cylinders are widely used. Unfortunately, the use of disposable gas cylinders is both expensive and cumbersome, due to the requirements of safely containing and using compressed gases.
Alternative methods of test gas generation have been developed. Electrochemical gas generators, such as those disclosed in U.S. Pat. No. 5,395,501, are available for several gases including hydrogen sulfide, chlorine, and chlorine dioxide. Electrochemical gas generators are obviously limited to those gases that can be produced by an electrochemical reaction. As for any electrochemical reaction, the amount of product produced is linearly proportional to the current passing, as described by Faraday""s law, and the diluent gas flow can be readily controlled, electrochemical generators in principle can provide good control over the gas concentration. In practice, the effects of absorption, changes in electrolyte composition, competitive electrode reactions and errors in the gas flow control limit the accuracy of these devices.
Calibration methods have also been devised in which the test gas is periodically generated, under the control of a microprocessor or other controller within the gas detection instrument. This approach allows the instrument to perform a gas test on the instrument without the need for a human operator. For example, electrochemical gas generators are used by Analytical Technology Inc. of Oaks, Pa. 19456 (8 Page Technical Information Sheet, entitled A world of gases . . . A single transmitter) to provide test gas to automatically check the performance of gas detection instruments, and ensure that the sensors are responding within their specified limits.
Automatic calibration methods have also been described in the prior art. For example, U.S. Pat. Nos. 4,384,925, 4,151,738, 5,239,492 and 4,116,612 describe methods for automatic calibration of a gas detection instrument in which calibration gas is automatically applied to the sensors under the control of a microprocessor. However, in most of these examples, the source of the test gas is still a compressed gas cylinder.
Electrochemical gas generators have also been incorporated into gas detection instruments. See, for example U.S. Pat. No. 5,668,302 and PCT International application WO 98 25139 which describe the incorporation of an electrochemical gas generator into an electrochemical gas sensor. These electrochemical gas generators have been used for carbon monoxide sensors, though the test gas produced is hydrogen from the electrolysis of the aqueous sulfuric acid electrolyte. In this latter example, while the incorporation of the gas generator into the sensor has clear advantages, it would be desirable to test the sensor with the intended analyte gas, in this case carbon monoxide, instead of the surrogate gas, hydrogen. The electrochemical properties of hydrogen are very different from carbon monoxide, and the oxidation of the latter gas is highly dependent upon the catalytic nature of the electrode surface. As a result, a good response of the sensor to hydrogen does not guarantee that the sensor will perform equally well to carbon monoxide.
Permeation-tubes are another commonly used device for producing calibration gases. These devices typically contain a polymeric tube containing the liquefied gas in an air stream. As the internal gas concentration is constant and the external concentration is near zero, the diffusion rate of gas through the polymeric material will be constant for a constant temperature. While permeation tubes are widely used to provide laboratory test gases, their use in field calibrators is limited due to the requirement for very tight control (e.g. +/xe2x88x920.1xc2x0 C.) of the temperature for accurate gas generation. Despite the difficulties, portable instruments are commercially available, e.g. from Kin-Tech, Houston Tex. Also, the use of permeation ovens is generally limited to gases that are readily available in liquid form at ambient temperature.
The use of permeation ovens for permanent gases is possible, by frequently refilling the reservoir container, but this requirement restricts the applicability of this method and adds to both the experimental complexity and expense.
Chand recently described a modification to the permeation tube method to produce test gas concentrations with less dependence on the absolute temperature than conventional permeation devices. See U.S. Pat. No. 4,399,942. Various injection methods are used for producing gas mixtures in laboratory scale, but these methods are not often used for field calibration. See Gas Mixture, Preparation and Control, G. O. Nelson, Lewis Publ., Boca Raton, Fla. (1992).
In another approach, U.S. Pat. No. 3,960,495 described a test gas system for combustible gas sensors, whereby a small amount of combustible gas is constantly generated by evaporation of a combustible material in the vicinity of the sensor. A decrease in the constant background signal indicates failure of the sensor. However, as the vapor pressure of most organic liquids depends very strongly on temperature, the concentration of test gas generated will fluctuate with temperature and as a result, the output signal will also fluctuate with temperature.
Alternatives to using test gas have also been investigated. For infrared-based gas sensors, various approaches have been tried including the use of optical filters. See U.S. Pat. No. 5,616,823, wherein the change in light intensity reaching the detector on inserting a filter of known absorbance into the light path, is used to calibrate the instrument. Methods have also been developed to assess the status of electrochemical gas sensors based on applying or perturbing the bias potential. Examples of this approach have been described for exhaust gas oxygen sensors in U.S. Pat. No. 5,558,752 and for toxic gas sensors in U.S. Pat. Nos. 5,202,637 and 5,611,909.
All of these alternative methods test only one aspect of the function of the gas sensor, and if the sensor is failing in this aspect then the test will identify the problem. However, these tests typically fail to test all aspects of the gas detection process. For example, an optical filter will test the change in absorbance of infrared light reaching the detector, but it will be unable to test whether the gas path into the sensor is accessible. Similarly, electrical perturbations to an electrochemical sensor test the integrity of the electrode connections, but would fail to detect a blocked membrane or other component in the gas diffusion path. Clearly, these substitute tests have value, but they fail to provide the reliability that the sensor is operating correctly, obtained by testing the gas sensor with the intended analyte gas.
A gas generator has been disclosed by the present inventor in European application EP 890 837 A2 for checking a carbon monoxide gas sensor. The gas generator disclosed therein includes a resistor or resistor array with a coating containing an oxalate salt. By applying an electrical potential to one of the resistors, a rise in temperature occurs until carbon monoxide is produced as a result of thermal decomposition of the oxalate salt. The system requires temperatures in the range of about 150 to 300xc2x0 C. thereby necessitating a large power supply and effective thermal insulation from the rest of the gas detection instrument. Also, it discloses solely the production of carbon monoxide.
There are many so-called xe2x80x9cwet chemicalxe2x80x9d methods available for the laboratory scale production of gases, which are well known (The Merck Index, 12th Edition, (1996); W. M. Latimer, J. H. Hilderbrand, xe2x80x9cReference Book of Inorganic Chemistry, MacMillan Co., New York, 1940). Using these methods a wide variety of gases can be prepared. The difficulty in using these types of reaction for test gas generation is to prevent the mixing of the reagents prior to use, and control the mixing of the reagents when test gas is required.
In spite of the foregoing prior art teachings, there remains a need for a single system that can produce several different types of gas. The use of multiple gas generation technologies adds both expense and complexity to the gas delivery system and probably also the gas detection equipment. As many gas detection instruments measure more than one type of gas, or similar instruments are available for different gas types by minor modification of the either the sensor or the instrument, a common gas-generating technology is advantageous. This system should be able to produce a concentration of test gas suitable for providing either a calibration or a bump test for a gas detection instrument. The equipment should be small, economical, simple and easy to use.
The above described need has been met by the present invention.
A method of generating test gases at the point of use, involving mixing of two or more chemical reagents together, which when combined result in the emission of the test gas. These reagents are mixed with a solid matrix material. The matrix material is selected such that the reagents are unable to react together at ambient temperature. Upon heating the mixture to a sufficiently high temperature that the matrix material melts, one of more of the reagents dissolves in the matrix, thus allowing the reagents to mix and produce the test gas. Related apparatus is disclosed. This system can be included within a gas detection instrument, or in a separate device intended to supply test gas to a gas detection instrument.
By holding the reagents in a solid matrix, the reagents will only mix if the temperature were raised above the melting point of the matrix. By placing this mixture of the matrix material together with the reagents in contact with a heating element, then the generation of the test gas can be controlled by controlling the electrical power to the heating element. Preferably, the two reagents should be solid, or high viscosity liquids for ease of handling. At least one of the reagents should be soluble in the matrix material, such that it will dissolve into the matrix, when the matrix is heated to above its melting point. Dissolution of the reagent will allow it to travel by diffusion processes to the other reagent(s) whereupon they will react and produce the test gas.
If the gas-generating element, comprised of the matrix material, reagents and suitably juxtapositioned heating element, is placed near the gas entry path of a gas detection instrument, then upon activation of the heater, test gas will be produced, which will then enter the gas detection instrument. Suitable positioning of the gas-generating element may involve inclusion within the gas detection instrument, inclusion within a calibration cup (a commonly used device which fits over part of the gas detection instrument, to deliver gas to the instrument during calibration), or as part of sample pump adapter. Alternatively the gas-generating element may be in a separate unit, and delivered to the gas detection instrument by conventional means, such as by flexible tubing. The test gas generated may enter the gas detection instrument by diffusion, or it may be blended with diluent gas, such as clean air or nitrogen, and then applied to the sensor. Alternatively, for instruments equipped with sample draw pumps, the test gas may be drawn into the instrument, preferably combined with ambient or filtered air. By varying the reagents and the matrix material gas-generating elements can be produced for many different types of test gas.
A preferred method of the present invention for testing gas detection instruments includes providing two reagents that would react with each other chemically to produce the test gas, except that mixing of the two together adequately is resisted, either because they are both in solid form and held by the matrix, or because they are a liquid and are immobilized in a matrix material. For convenience of reference herein, either of these situations, as well as other conditions under which one or more matrix materials serve to resist mixing of two or more reagents so as to resist premature meaningful chemical reactions which create test gases will be referred to as xe2x80x9cimmobilizingxe2x80x9d the reagent. The matrix material is subsequently heated until the matrix permits movement of one or more of the reagents resulting in generation of a gas responsive to a chemical reaction between the reagents and employing the gas to test the gas detection instrument. If desired, more than two reagents may be employed and more than a single test gas may be generated.
The matrix is preferably heated to facilitate dissolution or mobilization, or both, of one or more reagents. A second reagent may be immobilized in a second portion of the matrix material and if desired, an interposed portion of the matrix material may be provided to establish a physical barrier between the two reagent containing matrix portions. Corresponding apparatus is provided.
It is an object of the present invention to provide a test gas-generating system which resists chemical reagent interaction until it is desired to generate the test gas.
It is a further object of the present invention to provide such a system wherein at least one of the reagents is immobilized in a matrix material and is subjected to heating when it is desired to permit chemical interaction between reagents to generate one or more test gases.
It is a further object of the present invention to provide such a system which may generate the gas within the gas detection instrument or externally thereof.
It is yet another object of the present invention to provide efficient means for testing the effectiveness of a gas detection instrument with respect to one or more gases being monitored by the gas detection instrument.
It is yet another object of the present invention to provide such a system wherein microprocessors may be employed to control the operation of the test gas generation.
It is yet another object of the present invention to provide such a system which may employ reagents which are liquid or solid.
It is yet another object of the invention to provide a self-contained, thermally activatable test gas generator which is capable of generating one or more test gases.
It is yet another object of the present invention to provide such a test gas-generating system which may be utilized externally of or within a gas detection instrument or may receive the gas detection instrument within the test gas-generating system.